I'm not at my computer, but this should get you where you need to be. The loading bar goes from 0 to 1.0, so you can use interpolation to make it move smoothly between the values.
So something like
Object set width to lerp(progress, progress*maxWidth, 2*DT)
Where max width is however wide your progress bar will be at completion. Adjusting the 2 will change how quickly the bar moves. I don't remember the value construct uses for loading progress, but substitute that with "progress".